Light locus generation for automatic white balance

Abstract

A light locus of an imaging system is generated in a chromaticity space of two dimensions. The light locus represents a collection of candidate illuminants. The imaging system captures a gray-card image under each of N light sources to obtain N points in the chromaticity space, wherein N is a positive integer no less than three. Each point in the chromaticity space is described by a coordinate pair calculated from red (R), green (G) and blue (B) tristimulus values of the point. A second order polynomial function is calculated by curve-fitting the N points, and the light locus is generated to represent the second order polynomial in the chromaticity space. One of the candidate illuminants from the light locus is then identified as an illuminant for an image captured by the imaging system. A method for color transformation between two imaging systems in a chromaticity space is also described.

What is claimed is: 
     
         1 . A method for generating and utilizing a light locus of an imaging system in a chromaticity space of two dimensions, wherein the light locus represents a collection of candidate illuminants, comprising:
 capturing, by the imaging system, a gray-card image under each of N light sources to obtain N points in the chromaticity space, wherein N is a positive integer no less than three, and wherein each point in the chromaticity space is described by a coordinate pair calculated from red (R), green (G) and blue (B) tristimulus values of the point;   calculating a second order polynomial function by curve-fitting the N points;   generating the light locus to represent the second order polynomial in the chromaticity space; and   identifying one of the candidate illuminants from the light locus as an illuminant for an image captured by the imaging system.   
     
     
         2 . The method of  claim 1 , wherein, when N is equal to three, the N light sources are: D65, and Illuminant A according to the International Commission on Illumination (CIE) standard, and a light source whose spectral distribution approximates a blackbody radiator with a temperature range substantially between 2000 and 2500 degrees Kelvin (K). 
     
     
         3 . The method of  claim 2 , further comprising:
 calculating an upper bound of the light locus with respect to color temperature in the chromaticity space based on a horizontal coordinate value obtained under the D65 light source.   
     
     
         4 . The method of  claim 2 , further comprising:
 calculating a lower bound of the light locus with respect to color temperature in the chromaticity space based on a horizontal coordinate value obtained under the light source whose spectral distribution approximates the blackbody radiator with the temperature range substantially between 2000 and 2500 degrees K.   
     
     
         5 . The method of  claim 1 , wherein after calculating the second order polynomial function, the method further comprises:
 capturing, by the imaging system, the gray-card image under one or more additional light sources to obtain one or more additional points in the chromaticity space; and   verifying the light locus by determining whether the one or more additional points lie within a threshold distance from the light locus.   
     
     
         6 . The method of  claim 5 , wherein the one or more additional light sources include one or more of: daylight light sources and tungsten light sources. 
     
     
         7 . A method for color transformation between two imaging systems in a chromaticity space of two dimensions, comprising:
 calculating a first set of points in the chromaticity space from a first set of tristimulus values obtained by a first imaging system which captures color images of objects under a set of light sources, wherein each tristimulus values include a red (R) value, a green (G) value and a blue (B) value;   calculating a second set of points in the chromaticity space from a second set of tristimulus values obtained by a second imaging system which captures color images of the objects under the set of light sources, wherein each point in the first set of points has a corresponding point in the second set of points, and corresponding points are obtained from a same object captured by the two imaging systems under a same light source;   estimating a color transformation matrix that transforms the first set of tristimulus values to the second set of tristimulus values for each pair of the corresponding points; and   applying the estimated color transformation matrix to convert color signals generated by the first imaging system.   
     
     
         8 . The method of  claim 7 , further comprising:
 converting, using the estimated color transformation matrix, a first light locus of the first imaging system to a second light locus of the second imaging system, wherein each of the first light locus and the second light locus represents a collection of candidate illuminants in the chromaticity space; and   identifying one of the candidate illuminants in the second light locus as an illuminant for an image captured by the second imaging system.   
     
     
         9 . The method of  claim 7 , wherein the estimated color transformation matrix is a 3×3 matrix, the method further comprising:
 setting one element of the estimated color transformation matrix to a fixed constant; and 
 calculating the estimated color transformation matrix by minimizing an error metric in the chromaticity space. 
 
     
     
         10 . The method of  claim 7 , wherein coordinate values in the chromaticity space are invariant of: luminance of the set of light sources, non-uniform lighting, exposure errors and lens shading. 
     
     
         11 . The method of  claim 7 , wherein the set of light sources includes at least one light source selected from a group including: D65 and Illuminant A according to the International Commission on Illumination (CIE) standard, and a light source whose spectral distribution approximates a blackbody radiator with a temperature range substantially between 2000 and 2500 degrees Kelvin (K). 
     
     
         12 . A system which generates and utilizes a light locus in a chromaticity space of two dimensions, wherein the light locus represents a collection of candidate illuminants, comprising:
 an image sensor to capture a gray-card image under each of N light sources to obtain N points in the chromaticity space, wherein N is a positive integer no less than three, and wherein each point in the chromaticity space is described by a coordinate pair calculated from red (R), green (G) and blue (B) tristimulus values of the point;   a processor coupled to the image sensor, the processor operative to:
 calculate a second order polynomial function by curve-fitting the N points; 
 generate the light locus to represent the second order polynomial in the chromaticity space; and 
 identify one of the candidate illuminants from the light locus as an illuminant for an image captured by the imaging system. 
   
     
     
         13 . The system of  claim 12 , wherein, when N is equal to three, the N light sources are: D65, and Illuminant A according to the International Commission on Illumination (CIE) standard, and a light source whose spectral distribution approximates a blackbody radiator with a temperature range substantially between 2000 and 2500 degrees Kelvin (K). 
     
     
         14 . The system of  claim 13 , wherein the processor is further operative to:
 calculate an upper bound of the light locus with respect to color temperature in the chromaticity space based on a horizontal coordinate value obtained under the D65 light source.   
     
     
         15 . The system of  claim 13 , wherein the processor is further operative to:
 calculate a lower bound of the light locus with respect to color temperature in the chromaticity space based on a horizontal coordinate value obtained under the light source whose spectral distribution approximates the blackbody radiator with the temperature range substantially between 2000 and 2500 degrees K.   
     
     
         16 . The system of  claim 12 , wherein after calculating the second order polynomial function, the processor is further operative to:
 verify the light locus by determining whether one or more additional points in the chromaticity space lie within a threshold distance from the light locus, wherein the one or more additional points are obtained from the gray-card image captured under one or more additional light sources.   
     
     
         17 . The system of  claim 16 , wherein the one or more additional light sources include one or more of: daylight light sources and tungsten light sources. 
     
     
         18 . A system operative to perform color transformation from a reference system in a chromaticity space of two dimensions, comprising:
 an image sensor to capture color images of objects under a set of light sources; and   a processor coupled to the image sensor, the processor operative to:
 calculate a target set of points in the chromaticity space from a target set of tristimulus values obtained from the captured color images of the objects under the set of light sources, wherein each tri stimulus values include a red (R) value, a green (G) value and a blue (B) value; 
 calculate a reference set of points in the chromaticity space from a reference set of tristimulus values obtained by the reference system which captures color images of the objects under the set of light sources, 
 wherein each point in the reference set of points has a corresponding point in the target set of points, and corresponding points are obtained from a same object captured by the system and the reference system under a same light source; 
 estimate a color transformation matrix that transforms the reference set of tristimulus values to the target set of tristimulus values for each pair of the corresponding points; and 
 apply the estimated color transformation matrix to convert color signals generated by the reference system. 
   
     
     
         19 . The system of  claim 18 , wherein the processor is further operative to:
 convert, using the estimated color transformation matrix, a reference light locus of the reference system to a target light locus of the system, wherein each of the reference light locus and the target light locus represents a collection of candidate illuminants in the chromaticity space; and   identify one of the candidate illuminants in the target light locus as an illuminant for an image captured by the system.   
     
     
         20 . The system of  claim 18 , wherein the estimated color transformation matrix is a 3×3 matrix, the processor is further operative to:
 set one element of the estimated color transformation matrix to a fixed constant; and 
 calculate the estimated color transformation matrix by minimizing an error metric in the chromaticity space. 
 
     
     
         21 . The system of  claim 18 , wherein coordinate values in the chromaticity space are invariant of: luminance of the set of light sources, non-uniform lighting, exposure errors and lens shading. 
     
     
         22 . The system of  claim 18 , wherein the set of light sources includes at least one light source selected from a group including: D65 and Illuminant A according to the International Commission on Illumination (CIE) standard, and a light source whose spectral distribution approximates a blackbody radiator with a temperature range substantially between 2000 and 2500 degrees Kelvin (K).